The feature was protected by bug 88804 but not enabled by default for CMake.
Created attachment 147054 [details] Patch
Comment on attachment 147054 [details] Patch r=me sorry about forgetting cmake.
Comment on attachment 147054 [details] Patch Clearing flags on attachment: 147054 Committed r120064: <http://trac.webkit.org/changeset/120064>
All reviewed patches have been landed. Closing bug.